The following abbreviations are used in the description below:                3GPP third generation partnership project        AMR adaptive multi-rate        BER/BLER bit error rate/block error rate        CMR codec mode request        CN core network        DL downlink        DTX discontinuous transmission        e-NodeB E-UTRAN Node B (base station or other access node) of an LTE        E-UTRAN evolved UTRAN (also known as LTE or 3.9G or SAE)        GERAN GSM EDGE radio access network        HARQ hybrid automatic repeat request        HSPA high speed packet access        IP Internet Protocol        LA location area        LTE long term evolution of 3GPP        MGW mobility gateway        PCM pulse code modulation        PCRF policy and charging rules function        PDCP packet data control protocol        RAB radio access bearer        RRC radio resource control        RTP real-time transport protocol        SID silence descriptor        TFO tandem free operation        TrFO transcoder free operation        UE user equipment (e.g., mobile equipment/station)        UL uplink        UMA unlicensed mobile access (also generic access network GAN)        UMTS universal mobile telecommunications system        UTRAN UMTS terrestrial radio access network (also known as 3G)        VoIP voice over Internet protocol        WB wideband        WCDMA wideband code division multiple access        WLAN wireless local area network        
3GPP is standardizing the long-term evolution (LTE) of the radio-access technology which aims to achieve reduced latency, higher user data rates, improved system capacity and coverage, and reduced cost for the operator as compared to legacy systems. LTE is a fundamental redesign of UTRAN (3G) employing IP packet transmission and as such many of the particular implementations in UTRAN are not directly transferable to E-UTRAN. One such implementation relevant to these teachings is the adaptation of AMR codec bit rate control, to optimize the payload size in RTP packets so as to better use the limited radio resources available to the system. The legacy implementations of AMR rate adaptation is alternately implemented by link quality (in GSM/GERAN) or by radio network controller RNC bearer control (in WCDMA/UTRAN). First are detailed some prior art implementations of AMR codec bit rate control.
Slow rate control is used in the global system for mobile telecommunications (GSM) system. Cellular operators often will apply a change between half rate and full rate coding at certain predetermined times of the day in order to gain cell capacity. This interfaces with common restrictions on users, for example where a subscriber agreement allows for a flat rate monthly fee subscribers to use a certain number of minutes during peak traffic periods (e.g., between 7 AM and 8 PM) and a higher number of minutes during off-peak periods. The operator will impose the half-rate during the peak periods to handle a greater volume of traffic and the full rate during the off-peak period to provide a higher connection quality when the radio resources are expected to be less in demand. For the case where there is an ongoing call during the rate changeover, typically the channel rate change is carried out by an intra-cell channel mode handover.
AMR link adaptation controlling AMR bit rates is used in GSM (GERAN). The GERAN radio interface does not support fast power control, but link adaptation is done using inband CMR signaling. The worst radio link (caller or callee) controls the bit rate for a voice call. A set of four modes from eight possible modes is controlled by the two bits using inband signaling. There is a 40 ms minimum delay between the mode changes. The base station controller BSC (functionally equivalent to a radio network controller RNC in the UTRAN system) serves as the master for changes, while the UE and TRAU (transcoder/rate adaptation unit) work as slaves for it.
AMR is also used in WCDMA systems. To gain coverage, cell capacity or to statistically lower the packet-based transport radio and core network load, a selected set of rates (from 1 to N) can be given to the transcoder or to the UE or to both (where UL and DL rates are controlled separately). This can be controlled during RAB establishment by providing the set of permitted rates to the UE (for UL rate control) and to the transcoder (for DL rate control and initialization) that the rate control function will be using. So in WCDMA the RNC controls the overall AMR bit rates and thus the capacity of cells under the RNC's control. In WCDMA the transcoder is not part of the wireless system as in UTRAN, but part of the core network CN. Thus the CN can include or exclude the transcoder from the call path based on service criteria. What is termed transcoder free operation (TrFO) is therefore possible, which gives the associated benefits such as reduced transmission costs, higher speech quality in UE-UE calls, and reduced transcoder resources, for example.
In UTRAN the AMR bit rate control in the air interface is implemented by use of transport formats of the associated RAB. Restrictions for the operation of the AMR in UTRAN are not the same as in GSM: all eight rates (and discontinuous transmission DTX, where the UE ‘sleeps’ in a reduced power mode) can be within the configuration (i.e. the active codec set), and the rate can theoretically change at any time between all eight rates or the UE can enter the DTX mode using SID frames (e.g., frames containing only comfort noise parameters). However, in the case of TFO-TrFO interoperation between UTRAN and GERAN systems, up to four AMR modes plus SID are allowed on the UTRAN side. Inband CMR signaling from the GERAN is converted to rate control messages towards the UTRAN in the MGW to enable the LA function in the 3G-to-GSM direction. For a proper LA operation in the GSM-to-3G direction, the UTRAN must follow the 40 ms delay rule of the GSM LA while doing rate control towards the GSM system.
One relevant standard in that regard is 3GPP TS 26.103 v7.0.0 (2007-06) entitled TECHNICAL SPECIFICATION GROUP SERVICES AND SYSTEMS ASPECTS; SPEECH CODEC LIST FOR GSM AND UMTS; (Release 7). That reference discloses that the active codec mode is selected from the active codec set (ACS) by the network. This codec mode adaptation, also termed rate control, can be performed for the UMTS AMR-WB every 20 ms for the downlink traffic channel, but only every 40 ms for the uplink traffic channel by going to another codec mode within the ACS. The UE selects at call setup one of the two possible phases for codec mode adaptation (odd or even frames). During the call, changes of the codec mode in the uplink direction are only allowed in this selected phase. Rate control commands received in the downlink direction are considered at the next possible phase. By this definition, the UMTS AMR-WB codec type is TFO and TrFO compatible to the full rate FR AMR-WB, the optimized half rate OHR AMR-WB and optimized full rate OFR AMR-WB and the UMTS AMR-WB codec types.
The codec modes in uplink and downlink at one radio leg may be different. In tandem free operation or transcoder free operation, both radio legs (uplink A and downlink B) are considered for the optimal selection of the active codec mode in each direction (uplink A and then downlink B, respectively vice versa) by a “Distributed Rate Decision” algorithm. The worst of both radio legs determine the highest allowed codec mode, respectively the maximally allowed rate. All rate control commands are transmitted inband on the Iu and Nb interfaces and out-of-band on the radio interface.
The active codec set is selected at call setup or reselected during the call. It consists of three or four codec modes at a given time, selected from the allowed configurations. The selection of the configuration may be constrained by the network to consider resources and radio conditions. The active codec sets in uplink and downlink are typically identical.
Power is an important radio resource in a WCDMA system, and different AMR modes require different amounts of transmission power. The WCDMA air-interface has an in-built link quality control system that includes fast power control and some kind of quality based outer loop power control. The link quality control ensures that sufficient quality is maintained for each radio link. The quality target will be met even under difficult propagation conditions but at a cost of high transmission power. Instead, the selection of mode is based on the loading level in the system. There is no need for adapting the AMR modes to handle link quality variations like in a GSM system and on the power consumption of individual radio links. Thus, the admission control and rate control algorithms within the radio network generally govern AMR mode changes.
In the case of HSPA VoIP, cellular network based rate control is exercised. The CMR bits are however utilized as defined in the RTP specification, but the network has no control over it.
In UMA there is AMR VoIP rate control, but in that case, the rate is only matched with the AMR rate in the GERAN cell i.e., there is no local rate control in UMA but the far end connection will decide the active AMR rate. Additionally, the need for rate control depends on whether the network implements the transcoding into PCM. If AMR is not transcoded and is instead sent through an adaptation layer implemented in the media gateway, both ends of the VoIP path are following the AMR rate of the GERAN radio frequency RF link. The WLAN link and the UMA terminal merely adjust to the RTP header AMR bit rate information.
RTP AMR frame header supports the CMR signaling. Unlike GSM, there are no limitations for minimum period between rate changes and there is no subset of AMR rates used in the control process.
In LTE all transmission is packet oriented, and therefore the radio interface protocol layers do not recognize specific packets as AMR. The radio interface uses adaptive coding and HARQ mechanisms to ensure the required BER or BLER. As smaller packets improve the link BLER, it would be advantageous to include AMR bit rate control also to LTE as it is also supported by the RTP header. The biggest gain to the LTE comes however from lowering the overall AMR bit rates, as this enables more active VOIP calls supported in an LTE cell. The need for bit rate control arises when the load is increased to the level of cell capacity. In LTE there is no mechanism for AMR rate control to handle high voice call traffic situations and enabling the cell to serve more VOIP users. Note that in addition to get higher overall capacity to LTE, it is possible to improve BLER of terminals experiencing bad radio conditions prior to the handover. When the E-NodeB measurements indicate worse conditions, the change of AMR rate for the terminal will improve BLER and improve voice quality. The teachings below detail a way of implementing AMR bit rate control in a manner that is compatible with LTE.